There has been considerable effort devoted toward development of nitride semiconductor light-emitting diodes (LEDs) for efficient solid-state lighting (SSL) and for full-color displays.
In the case of SSL, white light is generated by a blue LED through phosphor conversion of blue light into yellow; and this combination of blue and yellow appears white. Accordingly, the LED package typically includes a phosphor element which absorbs some of the blue emission from the nitride LED, and re-emits at yellow wavelengths. In order for the white emission to appear uniform over all viewing angles, the phosphor is distributed very uniformly. Inkjet printing and spin-on deposition technologies have been developed for phosphor application. Likewise, direct generation (phosphor-free) of white light has been demonstrated for chirped-quantum-well (QW) structures, for which the LED active region contains several QWs of different composition and thickness, each designed to emit at a wavelength whose combination appears white. Similarly, the multiple emission wavelengths may be accomplished by stacking two LED structures and placing a tunnel junction between them.